Pickling method for profiles, rolled strips, and sheets made of aluminium alloys

ABSTRACT

In a method for cleaning a non-machined aluminum alloy product involving alkaline degreasing and treatment thereof after alkaline pickling with an acidic solution, the aluminum alloy product undergoes (a) initial acid cleaning before alkaline degreasing or (b) alkaline degreasing, followed by a subsequent acid rinse, alkaline treatment anew and an additional acid rinse thereafter. The aluminum alloy product may be a rolled aluminum alloy strip, a rolled aluminum alloy sheet and an aluminum alloy profile.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation of PCT/EP2018/084591, filed Dec. 12, 2018, which claims the benefit of European Patent Application No. 17206709.2, filed Dec. 12, 2017, the entire teachings and disclosure of which are incorporated herein by reference thereto.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a cleaning method for aluminum alloy products with the aim of producing a uniform surface appearance and excellent corrosion resistance and to aluminum alloy products produced by using the method according to the invention.

BACKGROUND OF THE INVENTION

In particular in the rolling steps involved in the production of aluminum alloy strips from aluminum alloy ingots, rolling oil or rolling emulsion is used, which, owing to the rolling steps involved, is incorporated into the surface of the aluminum alloy strip together with other particles. After rolling or between individual rolling passes, heat treatment processes, preferably annealing, for example intermediate annealing or final annealing, can also be performed for modeling the aluminum alloy strip into a specific microstructural state or for customizing the desired mechanical characteristics. The annealing of an aluminum alloy strip leads to an accumulation of alloying components such as zinc, silicon, copper or magnesium, in particular magnesium oxides, in areas of the strip close to the surface. As a result of this accumulation on the surface, regardless of whether it is due to heat treatment of the strip or due to the rolling process or in general due to the alloy composition, the aluminum alloy strip has a darker surface. Thus, even after degreasing, the aluminum alloy strip continues to have a dark surface appearance.

The impurities on the surface of the aluminum alloy strip consequently include dirt, metal abrasion and oil or oil decomposition products. In addition, the oxide layer on the surface of the aluminum alloy strip may have imperfections. It is a known fact that the deformation of aluminum materials at elevated temperature changes the surface properties by forming irregular microcrystalline structures close to the surface (Wear 206 [1997], 168).

To remove these impurities and to improve the surface quality, the aluminum alloy strip can undergo an acid pickling process. Particularly good results are obtained if the aluminum alloy strip first undergoes alkaline pickling or mild alkaline degreasing and in a subsequent step an acid rinse or acid pickling. Such a method is described in WO 2013/113598 A1 and is used on a large scale for the production of aluminum sheets for all possible applications, in particular for applications in the automotive industry. A lower quantity of impurities is removed from the surface through acid pickling alone than through alkaline pickling alone.

The surface structure of the aluminum alloy strip is cleaned by alkaline pickling. However, components of the oxide layer, such as magnesium oxides, that are insoluble in alkaline solutions continue to remain on the surface of the strip. These components are removed by the acid rinsing process (decapitation), whereby in particular protruding magnesium oxide structures on the surface of the strip are removed.

It has now been shown that a gray to grayish-brown irregularity can occur in the edge areas on both sides of the aluminum alloy strip. This phenomenon could be an annealing defect. Annealing defects of this kind can have a negative impact on the accessibility of the surface for subsequent wet-chemical treatment. Such defects therefore frequently occur in precisely this region.

This annealing defect could be caused by the fact that, during annealing of the coil, atmospheric oxygen from the edge region of the coil penetrates into the gaps of the coiled strip and, on the other hand, rolling oil, rolling oil components or the decomposition products thereof evaporate from the aluminum surface and possibly come into contact with the atmospheric oxygen in the edge regions of the strip, where a chemical or physical reaction then takes place and possibly leads to the gray to grayish-brown irregularity in the aluminum alloy strip.

It is also conceivable that these inhomogeneities involve different quantities and/or modifications of, for example, magnesium (oxide) or other alloying components such as zinc, silicon or copper which migrate to the surface during the thermal treatment process where they lead to a different lateral structure of the oxide layer due to the varying oxygen supply that penetrates the coil level and is present locally between the layers of the wound up coil. According to the invention, the term surface also refers to the layer with a thickness of preferably less than 0.5 μm, particularly preferably less than 0.2 μm, from the surface of the aluminum alloy product to the interior thereof (Z-axis). In particular in the case of aluminum alloy strips made from aluminum alloys with a high magnesium content, such inhomogeneities can be present on the surface despite alkaline degreasing and a subsequent acidic rinse. It is quite conceivable that this defect or inhomogeneity will only be made visible by a strong alkaline pickling solution. The strong pickling solution is nevertheless useful because it helps improve surface cleaning.

WO 2014/023283 A1 describes a method in which a particular type of defect, namely what is also known as soft spot formation, that usually leads to rejects only much later in the process can already be detected after the alkaline pickling process or after alkaline pickling with a (mild) acidic rinse. For this reason, a visual inspection is carried out after the above-mentioned pickling process, whereby soft spots can be detected and the component is thus declared as a reject at an early stage in the process. Such rejects are therefore not processed any further. Identifying rejects at an early stage saves costs. The component is preferably dried for the visual inspection. Due to the interruption resulting from the wet-chemical treatment baths, the pickling process must be repeated from the beginning after the visual inspection. The formation of soft spots, also called soft-spotting, is a phenomenon that may be visible as light-brownish spots in a compact black layer which is created during alkaline pickling by the cementation of copper. This is not considered as an annealing defect. Rather, WO 2014/023283 A1 indicates that soft-spotting is caused by local overheating during machining. The component referred to in this document is therefore always machined before pickling and anodization of said component after pickling is, in any case, intended. Furthermore, according to this document, alkaline pickling is always performed after alkaline cleaning. In this document, there is also no indication that the treated component has been annealed in the first place.

DE 10 2005 050556 B3 describes the method for cleaning or brightening soiled metallic surfaces. In this method, an object to be cleaned is first immersed in a diluted aqueous solution of an acid and a selected surfactant. Thereafter, it is treated with an aqueous alkali solution with a complexing agent and, optionally, also passivated in the final step. After treatment with an alkaline solution, no further treatment was performed with an acidic pickling or dipping solution.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is therefore to provide a method for obtaining aluminum alloy products (workpieces) that do not have this gray to grayish-brown edge or imperfection. This defect is clearly visible in aluminum alloy strips and other aluminum alloy products with a comparatively higher content of magnesium or aluminum alloy strips and other aluminum alloy products in which a thermal process (e.g. annealing) leads to a higher concentration of magnesium on the surface. Nevertheless, this process also presents advantages in applications involving other aluminum alloys.

It has now been established that the inhomogeneity mentioned above can be eliminated by exposing the aluminum alloy product sequentially to several jumps in the pH value. Consequently, the objective of the invention is achieved by a method for cleaning an aluminum alloy product which has not been machined, whereby the aluminum alloy product is cleaned beforehand with an acid, subsequently degreased with an alkaline solution and thereafter rinsed with an acid. Alternatively, the aluminum alloy product can be treated with an alkaline solution prior to acid cleaning (four-stage process).

During the entire method according to the invention, the aluminum alloy product is wetted with a treatment liquid. In this context, treatment liquid also refers to the rinsing liquid used in the optional steps involving rinsing baths.

The object of the invention also includes a method for providing an aluminum alloy product, whereby a rolled, strip-shaped aluminum alloy product is unwound from a coil that has undergone the previously described surface cleaning or surface modification according to the invention and is then optionally passivated and rewound into a coil.

The object of the invention also includes a method for providing an aluminum alloy product, in which an aluminum extrusion alloy is extruded to form a profile, subjected to the previously described surface cleaning or surface modification process according to the invention and subsequently optionally passivated on the surface and, if required, subjected to further shaping and optionally powder-coated.

Finally, the object of the invention is an aluminum alloy product which has been obtained by the method according to the invention and does not exhibit the described surface inhomogeneity despite the removal of impurities by pickling.

The sequence of these, at least three, pickling steps according to the invention is novel and surprisingly leads to better properties. The method according to the invention provides aluminum alloy strips and aluminum alloy sheets with an improved surface appearance, with improved corrosion test results, and it facilitates a faster rate of surface treatment of the metal strip, because the treatment time required for the alkaline degreasing process can be reduced without impacting the quality of the strip.

In the context of the present invention, the term aluminum alloy product refers to aluminum alloy strips, aluminum alloy sheets and aluminum alloy profiles. Aluminum alloy strips can be produced by rolling ingots or casting strips. Aluminum alloy profiles are produced by extrusion. In the following description of the invention, the term aluminum alloy strip is used to represent all these aluminum alloy products.

The method according to the invention involves an optimized pickling and surface cleaning process for aluminum alloy strips. This method is particularly advantageous for annealed aluminum alloy products that are meant to undergo an intensive pickling process. In the method according to the invention, the surface layers of the aluminum alloy strip with a concentration of magnesium/magnesium oxide or other acid-soluble alloying elements are removed in the initial acid cleaning stage that is conducted beforehand. The alkaline pickling solution removes the aluminum oxide matrix close to the surface and also removes aluminum and alkali-soluble alloying elements and intermetallic phases. The subsequent acid rinse removes alloying elements and intermetallic phases close to the surface, which are still present after the alkaline pickling stage.

The method according to the invention is conducive to improving surface properties and also helps save time and resources. The appearance of the strip is improved. The irregular lateral discoloration of the aluminum alloy strip is no longer present after the pickling process according to the invention. The entire surface of the aluminum alloy strip presents a homogeneous appearance. As a result of this homogeneous appearance, it can be concluded that every point of the surface of the aluminum alloy strip is prepared uniformly for subsequent processes. Local differences that indicate different properties of the surface are visibly removed by the method according to the invention and a constant result is thus achieved over the width and length of the strip.

The method according to the invention also achieves improved corrosion resistance in the filiform test. Surprisingly, the phosphating properties of the aluminum alloy sheet and its adhesive strength are improved. Furthermore, the loss of adhesive strength after weathering of the aluminum alloy strips that have undergone the pickling process in accordance with the invention is significantly lower than that of strips that have undergone the standard pickling process.

The pickling method according to the invention also presents advantages for aluminum alloy strips or aluminum alloy workpieces which do not possess the grayish-brown imperfection, because the strips or workpieces that have undergone the pickling process according to the invention display greater adhesive strength after weathering.

As a chemical reaction, the success of the pickling process always depends on the concentration of the reactants, the temperature and the contact time. The surprisingly better results achieved by virtue of the more effective alkaline pickling step now present commercial, ecological or quality advantages for several reasons: the treatment temperature can be reduced, the concentration in the degreasing solution can be reduced, the treatment time can be reduced (faster plant speed), or, within the same time, a greater quantity of impurities can be removed and a cleaner surface can be generated.

The source material of the method according to the invention is, for example, an aluminum alloy strip that is either produced by hot rolling an aluminum ingot and cold rolling or by continuous casting and cold rolling. The aluminum alloy strip may have been annealed. The aluminum alloy used may be of class AA 5xxx, for example AA 5005, AA 5083, particularly preferably AA 5182, AA 5754, AA 5454, AA 5251 and AA 5018, or of type AA 1xxx, for example AA 1050, AA 1110 and AA 1200, or of type AA 3xxx, for example AA 3003, AA 3004, AA 3005, AA 3103, AA 3104 and AA 3105, or of type AA 6xxx, for example AA 6016, AA 6014, AA 6005C, AA 6060, AA 6070 and AA 6451, or of type AA 7xxx and type AA 8xxx, for example AA 8006, AA 8011 and AA 8079, according to the International Alloy Designations of The Aluminum Association. The method according to the invention is particularly advantageous for strips of aluminum alloys with a higher magnesium content. The composition of such aluminum alloys is described in class AA 5xxx.

Furthermore, the method according to the invention is particularly advantageous for any aluminum alloys which have undergone annealing, preferably intermediate annealing, during production and/or which have been soft-annealed or re-annealed or solution-annealed in order to adjust their state. During thermal treatment, alloying elements diffuse to the surface where they may accumulate. The diffusion rate of the individual alloying elements can be different. Zinc, magnesium, silicon and copper, for example, have the fastest diffusion rate. After thermal treatment, therefore, the concentrations directly on the aluminum surface can be multiple times the concentration for this alloy. This applies to the annealing of aluminum strips both in the coil in the chamber furnace and annealing in the holding furnace.

The method according to the invention is, therefore, particularly advantageous for coil-annealed aluminum strips, in which, as mentioned above, inhomogeneities can occur on the surface of the aluminum strip due to different contents and/or modifications of, for example, magnesium (oxide) or other alloying components such as zinc, silicon or copper. During thermal treatment in the coil, the above-mentioned alloying components can migrate to the surface and lead to a different lateral structure of the oxide layer due to the varying oxygen supply that penetrates the coil level and is present locally between the layers of the wound up coil. The only important factor in this context is that the aluminum strip has undergone heat treatment in the coil during production. Discoloration or shading on the aluminum strip surface resulting from heat treatment in the coil, both in the form of intermediate annealing in the coil and final annealing in the coil, can be almost completely eliminated by the method according to the invention.

For example, an AlMg4.5 alloy, a typical AA 5xxx aluminum alloy for automotive sheet metal, can have a surface Mg concentration of 20% or more in the soft-annealed state. Likewise, an AlMgSi alloy with a nominal 0.5% Mg by weight, a typical AA 6xxx aluminum alloy for automotive sheet metal or for extrusion, can have a surface Mg concentration of 5% by weight and more in the solution-annealed T4 state, for example, in the whole alloy. Thus, the concentrations of the alloying components listed in the table provide only very little information about the composition that actually has to be chemically treated on the surface by the degreasing medium. The method according to the invention offers special advantages for all thermally treated alloys which contain, for example, Zn, Mg, Si, Cu or other rapidly diffusing alloying elements.

The material removed from the surface of the aluminum alloy strip after annealing by the method according to the invention is less than 100 nm, preferably less than 50 nm. Consequently, preferably only the surface layer with a concentration of magnesium and magnesium oxide is removed. An acidic cleaning solution is used for cleaning the aluminum alloy strip beforehand according to the invention. This cleaning solution can be an aqueous solution of at least one mineral acid, for example an aqueous solution containing sulfuric acid, nitric acid, phosphoric acid and/or hydrofluoric acid. These mineral acids may have a concentration of 0.2 to 10% by weight in the acidic pickling solution. The concentration of nitric acid can preferably be 0.2 to 5% by weight, particularly preferably 1.5 to 4% by weight, and that of sulfuric acid can preferably be 0.5 to 5% by weight, particularly preferably 1.5 to 3.5% by weight, in each case relative to the weight of the acid pickling solution. The acidic solution may contain 50 to 1,000 ppm fluoride, preferably 100 to 500 ppm, particularly preferably 200 to 400 ppm or 600 to 800 ppm fluoride. The pH level of the acidic cleaning solution may be adjusted for pickling applications and may contain other components. One or more surfactants in the aqueous pickling solution can aid in the degreasing of the aluminum alloy strip surface and increase the uniformity and rate of the pickling effect of the acid pickling solution in the initial cleaning stage. Preferably, one or more non-ionic or one or more anionic or cationic surfactants or a mixture thereof are used. Complexing agents can be used to aid in cleaning the strip. The initial acid cleaning step conducted beforehand may preferably take 0.5 to 15 seconds and particularly preferably 1 to 8 seconds or 1 to 5 seconds.

After initial acid cleaning of the aluminum alloy strip, an optional step involving rinsing can be included. Rinsing can be performed in one or more stages. Water can be used as a rinsing liquid. In addition to water, the rinsing liquid may contain a surfactant and, if required, other additives that aid in the rinsing effect.

The alkaline pickling solution or the alkaline solution for degreasing contains alkali metal hydroxides and/or alkaline earth metal hydroxides or carbonates. Alkali metal hydroxides are preferred. Sodium hydroxide is particularly preferred. The concentration of the alkali metal hydroxide or alkaline earth metal hydroxide is preferably 0.2 to 3% by weight, particularly preferably 0.4 to 2.5% by weight or 0.5 to 1.5% by weight, in each case relative to the weight of the alkaline pickling solution. The alkaline pickling solution may contain additives. Suitable additives include surfactants and complexing agents. The surfactants are preferably selected from non-ionic and anionic surfactants. Surfactants can be used in a concentration of 0.13 to 2% by weight, relative to the weight of the alkaline pickling solution. Suitable complexing agents are, for example, polyphosphates, phosphonates, gluconates, citrates and oxalates and may also be present as a mixture in the alkaline pickling solution. They can be used as sodium salts.

In large-scale applications, the dwell time of the aluminum alloy strip in the alkaline pickling solution can be 11 to 45 seconds, usually in a two-stage process (alkaline degreasing followed by acid treatment—known process or standard pickling). Surprisingly, it has now been found that the dwell time in the alkaline pickling solution can be reduced in the three-stage pickling process, thanks to the positive effects of initial acid cleaning on the result of the entire pickling stage. The dwell time in the alkaline pickling solution can thus be 1 to 25 seconds. The dwell time can, however, also be reduced to 1.5 to 15 seconds or 1.5 to 3 seconds or 1.5 to 6 seconds, if the process needs to be sped up. These dwell times allow sufficient cleaning, degreasing and pickling of the aluminum alloy strip surface in the method according to the invention. In a highly automated industrial process, the method according to the invention allows the production rate to be increased from 100 meters/minute to up to 150 meters/minute.

The dwell time of the aluminum alloy strip in the degreasing medium can also depend on the lye concentration and pH level. A more intense or sharper pickling implies a faster removal of the material including the impurities. It is known that the effect of an excessively strong alkaline solution can make inhomogeneities of the surface even more conspicuous. In the conventional two-stage degreasing process, the possibility of speeding up the process by more intense or sharper alkaline pickling is therefore limited. The method according to the invention involving at least three steps surprisingly increases the degrees of freedom. Optionally, the contact times can be extended accordingly, depending on the severity of the annealing defect.

Setting the temperature of the degreasing medium to 45° C. or 50° C. to 85° C., preferably 60° C. to 80° C. and particularly preferably 65° C. to 75° C. can also influence the dwell time of the aluminum alloy strip in the degreasing medium. The increased temperature leads to a higher reactivity of the degreasing medium and thus to a more intensive pickling treatment.

After alkaline degreasing, the aluminum alloy strip can preferably undergo rinsing. This may involve one or more rinsing steps. Water can be used as a rinsing liquid. In addition to water, the rinsing liquid may contain a surfactant and, if required, other additives that aid in the rinsing effect.

After the alkaline pickling treatment and the optional rinsing steps, the degreased aluminum alloy strip is treated with an acid pickling solution containing a strong mineral acid. The composition of the acidic solution for the initial cleaning procedure beforehand and the acid pickling solution can be identical. Suitable strong mineral acids are therefore nitric acid, sulfuric acid and phosphoric acid. The concentration of the strong mineral acid in the acid pickling solution may be 0.2 to 10% by weight, relative to the weight of the acid pickling solution. The concentration of nitric acid is preferably 0.2 to 5% by weight, particularly preferably 1.5 to 4% by weight, and the concentration of sulfuric acid is preferably 0.5 to 5% by weight, particularly preferably 1.5 to 3.5% by weight. The above acid concentrations allow good cleaning of the alkaline-pickled strips at high process speeds. As both acids sufficiently remove the surface impurities of the aluminum alloy strip during rinsing, a very clean and homogeneously pickled surface of the aluminum alloy strip can be reliably provided. Optionally, the pickling effect of the acid rinse can be adjusted by adding hydrofluoric acid or fluorides. Suitable fluoride concentrations are 50 to 1,000 ppm fluoride, preferably 100 to 500 ppm, particularly preferably 200 to 400 ppm or 600 to 800 ppm fluoride. The duration of this second acid rinse may preferably be 0.5 to 15 seconds, in particular 1 to 8 seconds and particularly preferably 1 to 5 seconds.

The aluminum alloy strip is preferably rinsed between the three pickling treatment stages and after the last acid rinse. Water or an aqueous solution can be used for this purpose. One, two or more rinses can be carried out between and after the pickling treatment stages. With the exception of the final rinse, the rinsing steps can optionally be omitted in order to achieve a more drastic change in the pH level. The final rinse is preferably carried out with fully demineralized water, particularly preferably at a high temperature, for example 60° C. to 95° C. The strip can then be dried.

As an alkaline solution for the alkaline treatment of the aluminum alloy product prior to initial acid cleaning that is to be performed beforehand as the first pickling stage in the four-stage process, the same alkaline pickling solution or, in other words, the alkaline degreasing solution can be used in the three-stage process. This procedure offers advantages for non-annealed aluminum alloy products. As an alternative to the alkaline pickling solution, a mildly alkaline cleaning agent or a neutral cleaning agent can be used for degreasing the aluminum alloy product in the four-stage process.

Both the three-stage and four-stage pickling of the aluminum alloy product are preferably carried out by spraying the aluminum alloy product with the respective pickling solutions and, optionally, by spray rinsing between the pickling treatment stages and after the last pickling stage. Aluminum alloy strips can also be passed through pickling baths with the respective pickling solutions, whereby rinsing baths can be arranged between the pickling baths.

The treatment solutions in the method according to the invention can therefore be applied by dipping, by flooding and squeezing or by spray treatment. At comparable chemical concentrations and treatment temperatures, spray applications generally require shorter contact times than dip treatment processes.

In the three-stage pickling method according to the invention, initial acid cleaning is therefore performed beforehand. Subsequently, the aluminum alloy strip can be rinsed. Alkaline pickling/degreasing is performed thereafter, optionally in conjunction with one or more rinses or rinsing baths. After this step, the aluminum alloy strip undergoes the final acid rinse, optionally once again in conjunction with one or more rinses or rinsing baths. The final rinse is preferably a rinse with fully demineralized water (κ<30 μS).

A four-stage cleaning process can also be implemented as an alternative to the three-stage cleaning process. The four-stage cleaning process involves the two-stage cleaning process according to WO 2013/113598 A1, which is performed first with an alkaline pickling solution, followed by acid rinsing. The component then undergoes alkaline treatment again and finally acidic treatment. The advantage of this procedure is that, by virtue of a shorter alkaline pickling treatment followed by acid rinsing, alkaline pickling starts much faster in the second run. As a result, the sum of the duration of the first and second alkaline pickling runs can be shorter than the duration of the single pickling run described in the process according to WO 2013/113598 A1.

In the four-stage method according to the invention, the treatment time or dwell time of the aluminum alloy product in the first alkaline pickling solution can be 1 to 12 seconds, preferably 1 to 5 seconds. The treatment time or dwell time of the aluminum alloy product in the first acid rinse can be 0.5 to 15 seconds, preferably 1 to 8 seconds. The treatment time or dwell time of the aluminum alloy product in the second alkaline pickling solution may be 1 to 12 seconds, preferably 1 to 5 seconds. The treatment time or dwell time of the aluminum alloy product in the second acid rinse may be 0.5 to 15 seconds, preferably 1 to 8 seconds.

According to a preferred embodiment of the four-stage method according to the invention, the treatment time of the first pickling process is only half the time of the second pickling process.

According to a further embodiment of the method according to the invention, the surface of the aluminum alloy strip can be passivated, for example by chromating, by chromium-free passivation based on zirconium and/or titanium or by passivation based on sol-gels, siloxanes or silanes, after the pickling treatment that involves at least three stages or after the four-stage process or after the final rinse and, if required, drying stage. Other passivation methods are also conceivable.

In this context, a no-rinse method for applying the surface passivation solution is preferable. Surface passivation simplifies subsequent process steps, such as joining the components with the aid of adhesives or by welding, phosphating or painting the surface, and also provides sufficient protection against influences that affect the surface quality of the strip. Surface passivation is preferably performed “inline” by implementing the cleaning or pickling methods according to the invention. “Inline” surface passivation can therefore be performed immediately in the same plant after the rinsing stage following the last acid rinse of the strip and without any need to wind up the strip before performing surface passivation. In this way, the surface condition of the aluminum alloy strip can be optimally preserved.

The aluminum alloy sheets produced according to the invention can be used for vehicle construction, offset printing, packaging and building construction purposes. They are used, for example, in car body construction, chassis construction, the construction of ships and yachts, architectural applications, lithographic plates, food and pharmaceutical packaging, tin cans and closures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photograph of an aluminum alloy strip coil with the previously described annealing defect after alkaline degreasing and acid rinsing according to WO 2013/113598 A1.

FIG. 2 is a photograph of an aluminum alloy strip coil of the same batch after complete elimination of the annealing defect by implementing a three-stage treatment process involving initial acid cleaning followed by alkaline degreasing and subsequent acid rinsing.

FIG. 3 shows the results of a corrosion test in a metal sheet obtained according to the invention and a comparison metal sheet made from the same aluminum alloy strip.

FIG. 4 depicts the curve of the mass fraction of magnesium on the surface of an aluminum alloy strip (AA 5182) after annealing and after the three-stage pickling method according to the invention. The magnesium content was measured by glow-discharge optical emission spectroscopy. The upper curve shows the measurement results after annealing and the lower curve shows the measurement results after the three-stage method according to the invention.

FIG. 5 shows the composition of the first 0 to 500 nm of two aluminum alloy strips over half the width of the strip-shaped aluminum alloy product.

FIG. 6 shows the content of further alloying components of the surface composition from 0 to 500 nm of two aluminum alloy strips over half the width of the strip-shaped aluminum alloy product.

FIG. 7 shows the composition of the first 0 to 500 nm of two aluminum alloy strips over the full width of the strip-shaped aluminum alloy product.

FIG. 8 shows the efficient removal of surface accumulations in aluminum alloy strips according to the invention in relation to the alkaline treatment time.

FIG. 9 shows the decrease in adhesive strength after weathering compared to an unweathered reference group in strips that have undergone pickling according to standard pickling procedures and according to the pickling method described in the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The aluminum alloy strip shown in FIG. 1 has a high magnesium content and a composition that corresponds to that of AA 5182. The defect described earlier is visible at the edges as an irregular wavy grayish-brown streak. This strip initially underwent alkaline pickling (0.5% NaOH, 1.5% of a degreasing composition of non-ionic and anionic surfactants and of complexing agents; contact time 45 seconds) and thereafter acid pickling (2% HNO₃+300 ppm F⁻; contact time 11 seconds). The strip shown in FIG. 2 is produced from the same batch as the strip in FIG. 1. The strip in FIG. 2 was additionally acid cleaned (2% HNO₃+300 ppm F⁻; contact time 11 seconds) according to the method described in the invention before being sent for pickling whereby the process time was reduced by half (23 seconds). No defect is visible on this strip.

Aluminum alloy strips, which were treated on the one hand according to the three-stage pickling method described in the invention (samples CV2, CV3, CV5) and on the other according to the known two-stage pickling method (samples CV1, CV4), were compared in a laboratory test. The same aluminum alloy was used in both cases and the treatment of the strips before pickling was also identical. The aluminum alloy in question is of type AA 5182. The sheets were degreased with an organic solvent before the comparative test was performed. The sheets were immersed manually in the baths. The measured values are shown in Table 1 below.

TABLE 1 Weight Pickling stage Pickling Weight Weight per unit H⁺- OH⁻ H⁺ area before after Weight area after before in in in Sample in m² pickling pickling loss pickling seconds seconds seconds CV-1 0.0470 134.06 g 133.82 g 0.24 g 5.0 g/m² 0 60 30 CV-2 0.0504 134.20 g 133.88 g 0.34 g 6.4 g/m² 12 60 12 CV-3 0.0472 133.31 g 133.08 g 0.23 g 4.9 g/m² 6 30 6 CV-4 0.0441 133.15 g 132.52 g 0.63 g 14.3 g/m²  0 180 60 CV-5 0.0470 133.58 g 133.39 g 0.19 g 4.0 g/m² 3 15 3

In the first row, “H+ before” indicates the acidic cleaning stage conducted beforehand, “OH—” stands for alkaline pickling and “H+” for the subsequent acidic rinse. The acidic pickling solution contained 5% by weight HNO₃ at room temperature. The alkaline pickling solution contained 2% by weight NaOH and 2% by weight of a degreasing agent composition at a temperature of 70° C.

After pickling, sheet CV-1 presents the surface appearance as shown in the photograph in FIG. 1 with an inhomogeneous surface color. Sample CV-2 shows a good result with a matte and homogeneous surface. Sample CV-3 shows the best result with a matte and homogeneous surface appearance. Sample CV-4 is better than sample CV-1, but still has an inhomogeneous appearance. The surface of Sample CV-5 presents a slightly inhomogeneous appearance with slight discoloration.

A corrosion test was performed for comparison purposes, whereby sheets of two aluminum alloy strips were subjected to what is known as the accelerated filiform corrosion test. Both sheets were taken from the same aluminum alloy strip, type AA 5182. The samples marked “Standard” underwent pickling with the known two-stage process; the remaining samples are samples obtained according to the invention. After pickling, the sample sheets were coated with a clear lacquer. The sheets were not passivated. In accordance with the accelerated filiform corrosion test, grooves with a width of one millimeter were made in the sample sheets. The sample sheets were treated with HCl and then kept at 40° C. and a relative humidity of 80% for five days. The results are shown in Table 2 below.

TABLE 2 Number of filaments Filiform no. Pickling Grooves per 50 mm Filaments/mm F = Freq. × L Standard 1 65.2 1.3 0.7 Standard 2 70.0 1.4 0.7 Standard 3 52.1 1.0 0.5 Standard 4 54.3 1.1 0.5 Standard 5 55.9 1.1 0.6 Standard 6 39.3 0.8 0.4 Invention 1 50.9 1.0 0.5 Invention 2 57.5 1.1 0.6 Invention 3 55.5 1.1 0.6 Invention 4 49.0 1.0 0.5 Invention 5 44.6 0.9 0.4 Invention 6 50.6 1.0 0.5 Standard Mean 56.1 1.1 0.6 Invention Mean 51.4 1.0 0.5

Table 2 shows that with 56.1 filaments per 50 mm the number of corrosion filaments is greater after the two-stage standard pickling process than after the pickling process according to the invention with 51.4 filaments per 50 mm. The sheet metal obtained according to the invention thus shows a better resistance to filiform corrosion. On the whole, the filiform factor is better for the metal sheets according to the invention. The effects of the test are shown in FIG. 3.

The pickling results of a four-stage pickling method according to the invention are shown in Table 3 below. The treatment of an aluminum alloy sheet of an AA 5182 aluminum alloy, as described in detail in the publication International Alloy Designations issued by The Aluminum Association, involved dipping in a bath with a first alkaline pickling solution, rinsing with water, dipping in a bath with a first acid rinse, rinsing with water, dipping in a bath with a second alkaline pickling solution, rinsing with water, dipping in a bath with a second acid rinse and rinsing with water. Both alkaline baths have the same composition and both acid rinses have the same composition. The alkaline pickling solution contains 2% by weight NaOH and 2% by weight of a degreasing agent composition at a temperature of 70° C. The acid rinse contains 5% by weight HNO₃ at room temperature.

The results in Table 3 below show that the four-stage method according to the invention facilitates a surprisingly shorter treatment time compared to the known two-stage process (V1, V2) and still leads to the good result of V1, in which the treatment times are comparatively very long. The long treatment time indicated for V1 is, however, uneconomical.

TABLE 3 1st pickling stage 2nd pickling stage Pickling Decapitation Pickling Decapitation [sec] [sec] [sec] [sec] Evaluation/comments Sheet 60 60 0 0 Very good pickling result with uniform, matte surface V1-1 Sheet 16 6 0 0 Poor pickling result, surface only slightly stripped V2-18 Sheet 60 60 60 60 Same as for Sheet V1-1 V3-2 Sheet 5 20 5 20 Poor result after the first pickling stage, hardly any stripping on the surface. V4-9 Slightly better result after the second pickling stage (light pickling). The surface is slightly more matte; the pickling time, however, is clearly too short. Sheet 2 15 20 15 Poor result after first pickling stage. Very good result after second pickling stage. V5-10 Matte and uniform surface comparable with V1 Sheet 1 15 20 15 Poor result after first pickling stage. Very good result after second pickling stage. V6-11 Matte and uniform surface comparable with V1 Sheet 1 15 15 15 Poor result after first pickling stage. Very good result after second pickling stage. V7-12 Matte and uniform surface comparable with V1 Sheet 1 3 15 3 Poor result after first pickling stage. Very good result after second pickling stage. V8-16 Matte and uniform surface comparable with V1 Sheet 1 3 15 3 Same as for Sheet V7-16; an intermediate rinse, however, was omitted. The result V9-19 is still very good.

Further pickling tests were performed in a spray booth. The quantity of material removed was determined for aluminum alloy strips consisting of four different alloys after said strips were treated using the pickling method according to the invention and the standard pickling process. Furthermore, both procedures involved alkaline pickling for 10 seconds and 20 seconds. The results in Table 4 show that, by implementing the pickling method according to the invention, less time is required for removing an equal quantity of material with impurities. The removal quantity (by pickling) was determined by differential gravimetric analysis before and after chemical treatment. Subsequently, the removal rate was determined for the treated surface.

TABLE 4 Initial acid Alkaline Subsequent Alkaline cleaning pickling acid rinse Removal pickling 3.0% H₂SO₄ 0.6% NaOH 3.0% H₂SO₄ quantity removal rate 400 ppm F⁻ 0.6% surfactant 400 ppm F⁻ per unit area per unit area Process Alloy Room temperature 60° C. Room temperature [g/m²] [g/m²s] Standard AA 6451 — 10 s 10 s 0.30 0.030 (short) Standard AA 6451 — 20 s 10 s 0.69 0.035 (medium) Invention AA 6451 10 s 10 s 10 s 0.76 0.076 (short) Invention AA 6451 10 s 20 s 10 s 1.10 0.055 (medium) Standard AA 6016 — 10 s 10 s 0.36 0.036 (short) Standard AA 6016 — 20 s 10 s 0.67 0.033 (medium) Invention AA 6016 10 s 10 s 10 s 0.49 0.049 (short) Invention AA 6016 10 s 20 s 10 s 0.82 0.041 (medium) Standard AA 6060 — 10 s 10 s 0.23 0.023 (short) Standard AA 6060 — 20 s 10 s 0.45 0.022 (medium) Invention AA 6060 10 s 10 s 10 s 0.39 0.039 (short) Invention AA 6060 10 s 20 s 10 s 0.80 0.040 (medium) Standard AA 5182 — 10 s 10 s 0.50 0.050 (short) Standard AA 5182 — 20 s 10 s 0.72 0.036 (medium) Invention AA 5182 10 s 10 s 10 s 0.76 0.076 (short) Invention AA 5182 10 s 20 s 10 s 1.18 0.059 (medium)

FIG. 5 shows a comparison of the effect of the method according to the invention on the composition of the alloying components in the surface layer of two aluminum alloy strips of the same alloy. The standard pickling process (alkaline pickling followed by acid rinse) was implemented for treating one strip, whereas the pickling method according to the invention was implemented to treat the other strip. The aluminum alloy strip treated by the standard pickling process shows the usual grayish-brown streaks which are located at the points marked on the strip treated by the standard pickling process in FIG. 1 (1=light, 2=dark, 3=light, 4=dark, 5=light, 6=dark, 7=light).

The concentrations of the alloying components across half the width of the strip (from the edge to the center of the strip) were determined to a depth of 500 nm from the surface of both aluminum alloy strips. The concentrations of the alloying components in the measured range are shown in FIG. 5, whereby Graph A shows the concentrations in the strip for which the standard pickling process was implemented and Graph B shows the concentrations in the strip for which the pickling method according to the invention was implemented. Only one half of the strip was analyzed across its width, as it can be assumed that a mirror image of the observations will be repeated on the other half of the strip.

The strip that has undergone pickling according to the method described in the invention (also see FIG. 2) shows a uniform distribution of the aluminum alloy components over the entire measurement area with a significantly reduced concentration of oxygen in contrast to the strip that has undergone pickling according to the standard process. No grayish-brown streaks occur in this strip.

The fluctuations in the proportions of the alloying components in the strip with grayish-brown streaks that has undergone pickling according to the standard process (also see FIG. 1) are significantly higher in contrast to the concentration of the alloying components measured in the strip that has undergone pickling according to the method of the invention.

The concentration of alloying components in the strip was determined by means of glow-discharge optical emission spectroscopy (GDOES). During the analysis, the elements of the surface are sputtered in a plasma in nanometric steps. The optical emission of the individual elements is then used to determine the element composition for each layer. The surface composition of the uppermost 500 nm was calculated by integrating the element compositions of all layers between 0 and 500 nm. The GDA 750 spectrometer from Spectruma Analytik GmbH was used to determine the concentration of the alloying components.

The graphs in FIG. 6 show the different concentrations of the other alloying components silicon, manganese, magnesium and copper on the surface of the aluminum alloy strips after treatment with the standard pickling process and after treatment with the method according to the invention. These analysis values also show an even distribution of the alloying components over the strip after pickling according to the method described in the invention in contrast to the strip that underwent pickling in the standard pickling process.

FIG. 7 shows the analysis results of the concentration of two aluminum alloy strips of an alloy composition AA 5018. The standard process was implemented for pickling one strip, while the method according to the invention was used for the other strip. With this alloy, no grayish-brown streaks appear on the strips. The advantage of using the pickling method according to the invention, however, is the higher efficiency of the alkaline pickling step during the same contact time, which is shown, for example, by the reduced oxygen content and the relatively reduced concentration of the alloying element magnesium on the surface.

FIG. 8 shows the surface composition of two aluminum alloy strips made of an aluminum alloy of type AA 6451. The strips underwent pickling according to the standard process and according to the method of the invention in a spray booth where the pickling agents were sprayed onto the strips. The concentrations of alloying components on the surface of the strips were determined by means of glow-discharge optical emission spectroscopy (GDOES). The sheets that underwent pickling according to the method of the invention achieve a lower concentration of alloying elements and oxides on the surface in a shorter period. A state of equilibrium is then achieved.

FIG. 9 shows the results of the adhesive strength analysis after 500 hours in the neutral salt spray test. The test parameters and procedure are described in DIN EN ISO 9227. The tensile lap-shear strength of bonded assemblies was determined for sheets of type AA 5182, one that underwent pickling according to the standard process and one that underwent pickling according to the method of the invention. The bonded assembly and subsequent lap-shear test was performed in accordance with “DIN EN 1465:2009-07” with an overlap length of L=10±0.25 mm, which deviates from the standard. FIG. 9 shows in particular the loss of adhesive strength after weathering, which is lower in the case of the sheet that underwent pickling according to the method of the invention than in the sheet that underwent pickling according to the standard process.

All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

1. A method for cleaning an aluminum alloy product which has not been machined, comprising: cleaning the aluminum alloy product beforehand with an acid, subsequently degreasing with an alkaline solution, and thereafter rinsing with an acid.
 2. The method according to claim 1, wherein the aluminum alloy product is treated with an alkaline solution before initial acid cleaning.
 3. The method according to claim 1, wherein the alkaline solution used for degreasing the aluminum alloy product has a temperature of 50° C. to 85° C.
 4. The method according to claim 1, wherein the aluminum alloy product is selected from rolled aluminum alloy strips, from rolled aluminum alloy sheets and from aluminum alloy profiles.
 5. The method according to claim 1, wherein the aluminum alloy product used is produced from an aluminum alloy of type AA 5xxx, AA 1xxx, AA 3xxx, AA 6xxx and AA 8xxx according to the International Alloy Designations of The Aluminum Association.
 6. The method according to claim 1, wherein the aluminum alloy product that is to be cleaned has undergone intermediate or final annealing.
 7. The method according to claim 1, wherein the aluminum alloy product is acid cleaned beforehand for 0.5 to 15 seconds, in particular for 1 to 8 seconds.
 8. The method according to claim 1, wherein alkaline degreasing of the aluminum alloy product lasts 1 to 25 seconds, in particular 1.5 to 15 seconds.
 9. The method according to claim 1, wherein the aluminum alloy product is acid rinsed for 0.5 to 15 seconds, in particular for 1 to 8 seconds.
 10. The method according to claim 1, wherein the surface of the aluminum alloy product is passivated after the acid rinse.
 11. An aluminum alloy product made of an aluminum alloy of class AA 5xxx, AA 1xxx, AA 3xxx, AA 6xxx and AA 8xxx according to the International Alloy Designations of The Aluminum Association, obtainable by any one of the processes of claim 1, wherein the aluminum alloy product does not possess any gray to grayish-brown irregularities or imperfections on the surface.
 12. The aluminum alloy product according to claim 11, wherein the aluminum alloy product has been annealed before it undergoes cleaning.
 11. The aluminum alloy product according to claim 11, wherein the magnesium content is homogeneously distributed across the width of the aluminum alloy product.
 14. The aluminum alloy product according to claim 11, wherein the aluminum alloy product is selected from rolled aluminum alloy strips, from rolled aluminum alloy sheets and from aluminum alloy profiles. 